<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        img {
            width: 100px;
            height: 100px;
            background-color: #f00;
        }
    </style>
</head>

<body>

    <div class="box">
        <img src="big1.jpg" alt="">
    </div>

    <ul>
        <li>
            <img src="1.jpg" alt="">
            <img src="2.jpg" alt="">
            <img src="3.jpg" alt="">
        </li>
    </ul>

    <script>
        var bigImgArr = ['big1.jpg', 'big2.jpg', 'big3.jpg'];

        var oImgs = document.querySelectorAll('ul img');
        var oBoxImg = document.querySelector('.box img');

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            oImgs[i].index = i;
            oImgs[i].onmouseover = function () {
                oBoxImg.src = bigImgArr[this.index];
            }
        }

    </script>

</body>

</html>